In high-speed reproduction machines, such as electrostatographic copiers and printers, a photoconductive member (or photoreceptor) is charged to a uniform potential and then a light image of an original document is exposed onto a photoconductive surface, either directly or via a digital image driven laser. Exposing the charged photoreceptor to a light image discharges the photoconductive surface thereof in areas corresponding to non-image areas in the original document while maintaining the charge on the image areas to create an electrostatic latent image of the original document on the photoconductive surface of the photoreceptor. A developer material is then brought into contact with the surface of the photoconductive member to transform the latent image into a visible reproduction. The developer material includes toner particles with an electrical polarity opposite that of the photoconductive member, causing them to be naturally drawn to it. A blank print substrate such as a sheet of paper is brought into contact with the photoconductive member and the toner materials are transferred to it by electrostatic charging of the substrate. The substrate is subsequently heated and pressed to permanently bond the reproduced image to the substrate, thus producing a hard print reproduction of the original document or image. Thereafter, the photoconductive member is cleaned and reused for subsequent print production.
Various sizes of print substrates are typically stored in trays that are mounted at the side of the machine. In order to duplicate a document, a print substrate with the appropriate dimensions is transported from the appropriate tray into the paper path just ahead of the photoreceptor. The substrate is then brought into contact with the toner image on the surface of the photoconductive member prior to transfer. However, a registration mechanism typically intercepts the substrate in advance of the photoconductive member and either stops it or slows it down in order to synchronize the substrate with the image on the photoconductive member. The registration mechanism also effects proper process direction (or longitudinal) alignment of the print substrate prior to delivery to the photoconductive member by correcting skew in the substrate. The registration mechanism also effects proper cross-process direction (or lateral) alignment of the print substrate prior to delivery to the photoconductive member by correcting lateral offset in the substrate.
The process of transferring charged toner particles from an image bearing member, such as the photoreceptive member, to an image support substrate, such as a print sheet, is accomplished at a transfer station. In a conventional electrostatographic machine, transfer is achieved by transporting an image support substrate into the area of the transfer station where electrostatic force fields sufficient to overcome the forces holding the toner particles to the photoconductive surface are applied to the substrate to attract and transfer the toner particles to the image support substrate. In general, such electrostatic force fields are generated via electrostatic induction using a corona generating device. The reverse side of the print sheet is exposed to a corona discharge while the front of the print sheet is placed in direct contact with the developed toner image on the photoconductive surface. The corona discharge generates ions having a polarity opposite that of the toner particles, thereby electrostatically attracting and transferring the toner particles from the photoreceptive image bearing member to the print sheet. An exemplary corotron ion emission transfer system is disclosed in U.S. Pat. No. 2,836,725.
Unfortunately, the interface between the image bearing surface and the print sheet is not always optimal. Particularly, with non-flat print sheets, such as print sheets that have already passed through a fixing operation (e.g., heat and/or pressure fusing), perforated sheets, or sheets that are brought into imperfect contact with the charge retentive surface, the contact between the sheet and the image bearing surface may be non-uniform, being characterized by gaps where physical contact fails. The toner particles tend not to transfer across these gaps, causing a print quality defect referred to as transfer deletion.
As described, the process of transferring development materials in an electrostatographic system involves the physical detachment and transfer of charged toner particles from an image bearing surface to a substrate via electrostatic force fields. In addition, other forces, such as mechanical pressure or vibratory energy, have been used to enhance the transfer process. The critical aspect of the transfer process focuses on applying and maintaining high intensity electrostatic fields as well as other forces in the transfer region to overcome the adhesive forces acting on the toner particles. Careful control of these electrostatic fields and other forces is required to induce the physical detachment and transfer of the charged toner particles without scattering or smearing the developer material.
The problem of transfer deletion has been addressed by various approaches. For example, mechanical devices that force the substrate into intimate and complete contact with the image bearing surface have been incorporated into transfer systems. Using this approach, blade arrangements have been proposed for sweeping over the back side of the substrate at the entrance to the transfer region. Alternatively, acoustic agitation or the use of vibratory energy has been disclosed as a method for enhancing toner release from the image bearing surface. Generally, systems using these methods also incorporate a resonator for generating vibratory energy, which is applied to the back of image bearing surface. Toner is thereby released from the image bearing surface despite the fact that the electrostatic charges in the transfer zone may be insufficient to attract toner from the image bearing surface to the substrate. These vibratory methods are not always effective. Therefore, other methods for improving the efficiency of transferring toner to the print substrate would be useful.